It is required that the cationic electrocoating compositions are stable at ambient temperature and quickly cure at an elevated temperature. In order to comply with the requirement, the curing temperature is very important. If the curing temperature is too high, energy consumption increases. If it is low enough to reach near ambient temperature, the compositions have poor storage stability. Accordingly, it is desired that the curing temperature is existent not near ambient temperature, but is not far from it.
Hitherto, the curing systems of the cationic electrocoating compositions, which have been employed, include a melamine curing system (e.g. U.S. Pat. No. 3,367,991, Japanese Kokai Publications (unexamined) 1256/1985 and 41571/1988); and a blocked isocyanate curing system (e.g. Japanese Kokoku Publications (examined) 34238/1980, 4978/1979, 22912/1981 and 39351/1986). The melamine curing system does not cure sufficiently at a suitable low temperature because the curing system proceeds in basic condition. In the blocked isocyanate curing system, it is difficult to cure at the desired temperature, because the blocked isocyanate curing agent is generally unblocked at relatively high temperatures. Although it is possible that the unblocking temperature lowers by selecting a blocking agent or a catalyst, such selection has some limits in view of coating composition ingredients, cured film properties and the like.